Several studies have looked at the effects of individual fibres and probiotics.

But few researchers had investigated the role of polyphenols which are common in much of what we consume - spices, teas, fruits and vegetables - or how polyphenols and fibres together help balance our gut microbes.

Spanish researchers led by Sonia Gonzalez tried to fill this gap.

The team asked 38 healthy adults questions about their diets and figured out which bacteria were present in the participants' stool samples.

Their analysis revealed that pectin, a compound in citrus fruits, lowers the levels of some helpful bacteria.

This is contrary to previous research on pectin alone.

"Our most novel finding was that white bread boosted Lactobacillus, a group of beneficial bacteria," Gonzalez added.

In addition to this surprising find, the study also revealed that when looking at effects of food on our "microbiomes", considering the whole diet, not just individual ingredients, is critical.

The research, funded by the Spanish Ministry of Science and Innovation, was published in the Journal of Agricultural and Food Chemistry.